Liquid metal matrix thermal paste

ABSTRACT

A liquid metal matrix thermal paste comprises a dispersion of non-reacting thermally conductive particles in a low melting temperature liquid metal matrix. The particles preferably are silicon, molybdenum, tungsten or other materials which do not react with gallium at temperatures below approximately 100° C. The preferred liquid metals are gallium and indium eutectic, gallium and tin eutectic and gallium, indium and tin ternary eutectic. The particles may be coated with a noble metal to minimize surface oxidation and enhance wettability of the particles. The liquid metal matrix thermal paste is used as a high thermally conducting paste in cooling high power dissipation components in conjunction with a conventional fluid cooling system.

This application is a division of application Ser. No. 07/389,131 filedAug. 3, 1989.

BACKGROUND OF THE INVENTION

The present invention relates to thermal paste and particularly relatesto a liquid metal matrix thermal paste containing fine thermallyconducting particles dispersed in a low melting temperature liquid metalmatrix and having high overall thermal conductivity. The thermal pasteis used to form a thermal joint between an electronic component, such asa chip, and a cooling system.

Modern very high performance electronic systems often require a highdensity of chips having many high power gates. These electronic systemsrequire cooling a high power density through a limited temperature dropfrom a device junction to a cooling system. In order to achieve thecooling requirement, the thermal joint from the chip to the coolingsystem must possess a high thermal conductivity. The thermal pastedescribed hereinafter provides the highest available thermalconductivity thermal joint between a chip or component and the coolingsystem obtained to date.

In practical applications, the joint, in addition to providing highthermal conductivity, must compensate for certain manufacturingtolerances inherent in any electronic assembly. For instance, whenassembling chips using the so-called controlled collapse circuitconnector technique (hereinafter referred to as C4), as a result ofvariations in the size and shape of the solder ball arrays used toconnect the chip to a printed circuit board, there are significantvariations in chip height and tilt relative to the printed circuitboard. The electrical connections, the C4 solder balls, are very fragileand the thermal joint must permit a certain amount of differentialmotion of the individual chips forming an electrical system assembly.Also, manufacturing tolerances cause variations of the gap between thechip and the cooling system. The combination of geometric variations andtolerances conflict with the goal of achieving good thermal conductionby causing a very thin paste layer to be manifest between certain of thechips and the cooling system. Despite the difficulty encountered, goodthermal conduction is still achievable by using a paste having high bulkthermal conductivity properties.

Using known conventional thermal paste has provided moderately good heattransfer plus moderate compliance. A commonly used paste contains amixture of zinc oxide in mineral oil. Such pastes have an upper limit ofthermal conductivity. Also, the liquid and particles tend to phaseseparate after many power on-off cycles. The conventional pastes relyupon the perculation of oxide particles in a low conductivity oil matrixfor thermal conductivity. The use of a low conductivity oil matrix isthe primary limiting factor in achieving high thermal conductivity. Thepresent invention provides for a high thermal conductivity of both theliquid matrix and the dispersed particles.

There have been many proposals to use liquid metal thermal joints,particularly mercury, which may be harmful both to humans and electroniccircuits. However, confining the liquid metal has been prohibitivelydifficult and requires a reduction of the thermal conductivity. Forinstance, U.S. Pat. No. 4,092,697 teaches a thermal joint pillow with aplastic film skin and macroscopic filling of liquid metal. Also, U.S.Pat. No. 4,323,914 teaches the coating of the entire chip with aparylene film coating and then adding a macroscopic metal joint to thecooling cap. Both of these patents degrade the thermal conductivity ofthe joint by including a poor thermal conductivity plastic film.

SUMMARY OF THE INVENTION

The present invention provides a liquid metal matrix thermal pastecontaining fine thermally conductive particles dispersed in a lowmelting temperature liquid metal matrix. Preferably, the particles aremetal or conductive non-metals selected to be non-reactive with theliquid metal matrix at low temperature in order to provide a semi-liquidfully compliant structure for an indefinite period of time. Theparticles are preferably tungsten, molybdenum, silicon or other metalsor high thermal conductivity materials which have a very low interactionrate with gallium, tin and indium at low temperature, i.e. attemperatures below approximately 100° C. The particles can also benon-metals having high thermal conductivity, such as diamonds. Theparticles can be coated to enhance wettability. The preferred liquidmetal matrix are gallium and indium eutectic alloys, gallium and tineutectic alloys, and gallium, indium and tin ternary eutectic alloys.

The liquid metal matrix thermal paste is formulated as described belowas a gallium-metal paste which permanently and indefinitely retains itssemi-liquid state under normal operating conditions and environment toserve as a compliant thermal interface. The resultant paste acts as ahigh thermal conductive medium which is formed into a desired shape toenhance thermal transfer. By retaining its semi-liquid state undernormal operating conditions without solidifying, the enhanced thermaltransfer via the paste is sustained during many on-off power cycles whenthe components of the cooling system expand and contract. Thedifferential expansion of the various components results in significantthermal stresses, which are particularly damaging at the componentinterfaces. A paste which solidifies over time, and therefore, does notprovide a compliant interface, will eventually crack after repeatedexpansion and contraction and cease to function as a good thermaltransfer medium.

A principal object of the present invention is, therefore, the provisionof liquid metal matrix thermal paste having a high thermal conductivitywhile remaining in a semi-liquid state for an indefinite period of time.

Another object of the invention is the provision of a liquid metalmatrix thermal paste having high thermal conductivity and adjustableviscosity.

A further object of the invention is the provision of a liquid metalmatrix thermal paste having high thermal conductivity for use as athermal joint between an electrical component, such as a chip, and acooling system, or between successive parts of a cooling system.

A still further object of the invention is the provision of a liquidmetal matrix thermal paste containing fine thermally conductingparticles dispersed in a low temperature liquid metal matrix which isstable for an indefinite period of time.

A still further object of the invention is the provision of a processfor preparing a liquid metal matrix thermal paste.

Further and still other objects of the invention will become moreclearly apparent when the following description is read in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional representation of thermally conductingparticles dispersed in a liquid metal matrix;

FIG. 2A is a cross-sectional representation of coated thermallyconducting particles dispersed in a liquid metal matrix as initiallymixed;

FIG. 2B is a cross-sectional representation of coated thermallyconductive particles dispersed in a liquid metal matrix after thecoating dissolves and when ready to be used;

FIG. 3 is a phase diagram of gallium-tin eutectic;

FIG. 4A is a cross-sectional representation of thermally conductiveparticles dispersed in a liquid metal matrix comprising 65 wt% Sn and 35wt% Ga at an elevated temperature;

FIG. 4B is a cross-sectional representation of thermally conductiveparticles dispersed in a liquid metal matrix comprising 65 wt% Sn and 35wt% Ga at a typical operating temperature;

FIG. 5A is a cross-sectional representation of thick coated thermallyconductive particles dispersed in a liquid metal matrix as initiallymixed; and

FIG. 5B is a cross-sectional representation of thick coated thermallyconductive particles dispersed in a liquid metal matrix when ready to beused.

DETAILED DESCRIPTION

Referring to the figures and to FIG. 1 in particular, there is shownschematically in cross-section a liquid metal matrix thermal paste. Thethermal paste contains thermally conductive particles 12, dispersed in aliquid metal matrix 14.

The liquid metal matrix thermal paste of the present invention comprisesfine thermally conductive particles dispersed in a low meltingtemperature liquid metal matrix. Preferred liquid metals are gallium andindium eutectic (75.4 wt%+24.6 wt%, MP 15.7° C.), gallium and tineutectic (86.3 wt%+13.7 wt%, MP 20.5° C.), gallium, indium and tinternary eutectic (21.5 wt%+16.0 wt%+62.5 wt%, MP 10.7° C.), and severalquarternary systems having melting points as low as 7° C. The ratios inthe parenthesis are the preferred mixture composition and melting pointof each alloy. The particles are metals or thermally conductivenon-metals. The particles are selected to be non-reactive with theliquid metal, such as gallium, at low temperatures of less thanapproximately 100° C. in order to provide a semi-liquid fully compliantpaste for an indefinite period of time. Preferred particles aretungsten, molybdenum, silicon or other particles having a lowinteraction rate with gallium at low temperatures, i.e. at temperaturesbelow approximately 100° C. Most of the particle materials, particularlysilicon, molybdenum or tungsten will have an oxide layer on the surfacewhich will impede wetting by a liquid metal. Therefore, referring toFIG. 2A, such particles are coated with a thin layer of a noble metal16, such as gold, palladium or silver, with the preferred metal beinggold. The thin noble metal layer, e.g. gold, dissolves in galliumwithout affecting the composition significantly thereby ensuring wettingbetween the exposed metal particle and the gallium. As shown in FIG. 2B,after the noble metal coating dissolves, gold-gallium intermetallics 18are formed and remain attached to the particles.

For optimum rheology of the thermal paste, spherical particles arepreferred. Commonly available tungsten and molybdenum powders are notspherical, but rather are of an irregular shape. In order to overcomethis problem, spherical metallic particles, such as copper, are coatedwith a barrier layer of tungsten or molybdenum which, in turn, is coatedby a thin layer of a noble metal, such as gold. Alternatively, thetungsten or molybdenum particles may be made spherical using any ofseveral known techniques. A preferred technique being plasma remeltingand solidification of the particles.

In order to maximize a solid filler while still maintaining lowviscosity, a bi- or tri-modal distribution of particles is used, whichincreases the higher thermal conductivity solid phase of the paste.

The liquid metal matrix paste is prepared using conventional pastemaking techniques. The preferred method is a combination of planetarymixing and a three roll mill dispersion. The mixing hardware should becoated with one of the numerous available coatings, such as tungsten,oxides, nitrides or carbides of any one of several elements, in order,to prevent contamination of the paste by dissolved elements.

The figures contained in the application depicting the paste are notdrawn to any particular scale but rather are presented in order tobetter describe the invention by means of illustrations.

Liquid Metal Thermal Paste Properties

Liquid gallium and its alloys have a thermal conductivity ofapproximately 28 W/MK. Solid tungsten has a thermal conductivity of 170W/MK. A dispersion of tungsten in a liquid gallium matrix will have athermal conductivity in the range between 28 and 170 W/MK. The exactvalue of the thermal conductivity will depend upon fraction anddispersion of the solid phase in the resultant paste.

The mathematical expression of the thermal resistivity of a jointcontains bulk thermal resistivity and surface thermal resistivity terms.The bulk thermal resistivity term is proportional to the jointthickness. The surface thermal resistivity term is dependent uponparticle size and surface finish. The surface thermal resistivity termis generally independent of the joint thickness provided it is a largevalue compared to the particle size term. The particle size, in turn,provides a limit on the minimum joint thickness. The particle size alsosets a scale for the magnitude of the surface effects. For a given jointthickness, reduction of a particle size as well as bimodal distributionwill diminish the surface effects contribution to thermal resistance.The liquid metal matrix thermal paste of the present invention withmicron sized particles where a typical surface term equals 2 microns ofbulk, or 2E-6M/200 W/MK=1E-8 KM² /W=0.000,1 K per W/cm². A thermalresistivity of this magnitude is considered excellent for mostapplications.

Liquid Metal Confinement

In order to prevent liquid metal from contacting nearby conductors orcomponents, polymeric or other barriers, including encapsulatingcompounds as fillers of the empty space between components, may be used.

Certain families of chips are required to be electrically isolated fromthe metal joint. Applying chemical vapor deposited amorphous carbon tothe chip at a moderate temperature, even after the chips have beenbonded to the common substrate of the multi-chip module, provides areliable pinhole free electrical insulator having a very low thermalresistivity. Alternatively, the application of thin dielectrics, such asSiO₂ or SiN, to each wafer before dicing also provides electricalisolation. In certain wafer processing steps SiO₂ is formed on the chipwhich then can be stripped off or left on the wafer as required.

In order to provide additional control of any excess liquid metal matrixthermal paste, a blotter, which can even be a fine mesh of a metal wire,is used.

It is well known that thermal resistivity is improved when the thermaljoint is thin. In order to form a thin joint, low viscosity thermalpaste is required. Pending U.S. patent application Ser. No. 07/161,880,describes forming of a 1 mil thick paste joint by simultaneouslyshearing and compressing conventional thermal paste between a chip and acooling unit. The same technique applies when using liquid metal matrixthermal paste provided the paste is not excessively stiff. A pastecomprising 35 wt% Ga-In-Sn ternary eutectic and 65 wt% tungsten in theform of 10 μm particle size has been thinned to approximately 3 to 5mils between two quartz slides while maintaining bubble-free interfaces.A paste containing 50 wt% tungsten in the form of 2.2 μm particle sizein 50 wt% Ga-In-Sn ternary eutectic was successfully thinned to 2 to 3mils. Further reduction in joint thickness is possible by optimizationof the thinning technique.

Semi Liquid Metal Matrix

In certain applications better results are attainable when a true liquidmetal is not employed. In these instances, a semi-liquid alloy having alarge liquidus-solidus range is useable. During formation of the thermalpaste or during assembly of the components with the cooling system, thethermal paste is sometimes heated to become a true liquid metal.However, when the heated liquid is used in a low or moderate temperatureapplication or operating environment, the liquid will become viscous.

Liquid metal matrix thermal paste according to the present invention, incertain applications, can be melted for only a short period of time. Thebrief melt period differs from solder reflow because the liquid metalremains as a liquid metal matrix thermal paste throughout the entiretemperature cycle.

A gallium-tin system is a preferred suitable system for the describedsemi-liquid metal matrix. FIG. 3 is a phase diagram of a gallium and tinsystem. The ordinate axis is the temperature of the mixture and theabscissa represents increasing percentage of tin. At temperatures abovethe curve 20 for particular ratios of tin and gallium, the mixture is aliquid. Below the horizontal line 22 the mixture is a solid. Forcombinations of temperature and tin-gallium ratios between the curves 20and 22 the mixture will be a semi-liquid of variable viscosity dependingupon whether the operating point at a particular ratio of tin andgallium is closer to the "liquid line" 20 or "solid line" 22.

For example, vertical line 24 represents a 65 wt% Sn - 35 wt% Ga alloy.At a temperature of approximately 21° C. the alloy is 63 wt% solid and37 wt% liquid. When the alloy is heated above 130° C. it issubstantially all liquid. If it is assumed that the thermal paste willbe applied to operate for use at an operating temperature of 70° C., thealloy will be 50 wt% solid and 50 wt% liquid. When such an alloy is usedinstead of a eutectic liquid matrix, the paste exhibits very lowviscosity during heating at a temperature in the range between 100° C.and 130° C. After the cooling hardware is assembled and operating at atemperature in the range between 50° and 80° C., the alloy will exhibitincreased viscosity. The described system will effectively prevent phaseseparation of the paste in service.

The system operation is shown schematically in FIGS. 4A and 4B. In FIG.4A thermally conductive particles 30 are dispersed in a liquid metalmatrix 32 comprising 65 wt% tin and 35 wt% gallium at a temperature of130° C. The metal matrix is substantially all liquid. When thetemperature is reduced to 70° C., tin 34 separates from the metal matrixwith some quantity of tin adhering to the particles 30. The metal matrixis partially solid and partially liquid thus increasing the viscosity ofthe paste.

Paste Thickening Technique

An alternative method of reducing the possibility of phase separation ofthe thermal paste and thus ensuring low viscosity of the paste duringassembly of the cooling structure and increased viscosity during normaloperating conditions is to increase the thickness of the wettable layeron the particles. As shown in FIG. 5A, as initially mixed the particles40, such as tungsten, dispersed in a liquid metal matrix 42, such asgallium, indium and tin ternary eutectic alloy, are coated with arelatively thick layer of a wettable metal, such as gold 44. As theliquid gallium, indium and tin react with the gold layer to produceintermetallic particles 46 as shown in FIG. 5B, the paste viscosityincreases and the possibility of phase separation is reduced.

Removal of Liquid Metal Matrix Thermal Paste

Rework generally entails the removal of thermal paste. The bulk of thepaste is removable by using a metal wool containing tin or copperfilaments. Thorough paste removal usually requires the use of ultrasonicagitation through a thin film of liquid without disturbing othercomponents. Thorough paste removal is also achievable by using a brushor a felt tip and a cleaning liquid. Such a liquid should benon-corrosive and not be a strong organic solvent. Preferred liquids areisopropanoyl or ethanol. Also preferred is hypercritical carbon dioxide.Alternative paste removal methods include brushing with metal wool whileusing tin powder as a sweeping agent and placing tin foil in contactwith the residual paste for removing gallium.

Surface Coating

In order to prevent the leaving of residue from the liquid metal matrixthermal paste such as by adhesion of indium to silicon dioxide glass orto a silicon chip or by metal amalgamation with a solid metal, anon-reactive coating is applied to the chip. The preferred coatingsinclude an ultra thin layer of Teflon or siloxane applied by sputteringand hydrogenated amorphous carbon applied by chemical vapor deposition.Alternatively, a thin tungsten coating applied either by chemical vapordeposition or by sputtering achieves similar results. Both tungsten andhydrogenated carbon coatings have withstood 100 hour exposure at 200° C.in contact with gallium, indium and tin eutectic liquid without anyinteraction being observed by various analytical techniques. As anadditional benefit, these coatings assist in the prevention ofthermomigration into and through a semiconductor material to preventunwanted doping of the chip.

Composite Particles

An alternative method of enhancing wetting, and therefore improvingadhesion between the liquid between the liquid metal matrix and thefiller metal particles, is by the use of composite metal particles. Forexample, Si - Au alloyed powder of 80 wt% Si-20 wt% Au composite, ifstabilized at a temperature in the temperature range between 200° C. and300° C., comprises a silicon phase with a gold phase dispersed therein.Such a powder is capable of being produced conventionally by gasatomization. When such a composite powder is dispersed in a galliumliquid matrix, the gallium will preferentially react with gold, whilenot reacting with the silicon. The result is enhanced wetting anddispersion possibly to a greater extent than surface coating alone. Ifthe gold is located inside the silicon particle, channels will be formedthrough the particles when the gold reacts with the gallium.

All Metal Matrix Thermal Paste Applications

The liquid metal matrix thermal paste has application with various typesof cold plates. The paste may also be used with a piston cooling hat,between the chip and the piston as well as between the piston andcylinder. The paste is usable as a thermal joint which is adjacent tothe chip, near but not adjacent to the chip and external to the coolinghat. In addition, the thermal paste is used with discrete components,particularly power components such as rectifiers, transistors andresistors.

EXAMPLES

Example I is a thermal paste of 50 wt% and 50 wt% Mo. The thermal pastewas used as a thermal joint between a single chip and a cooling system.The power density was 103 W/cm² through a total temperature differentialof 64° C. including both the joint and other thermal resistivities.

Example II is a thermal paste of 33 wt% Ga-In-Sn ternary eutectic and 67wt% tungsten comprising irregularly shaped particles having diameters inthe range between 5 and 20 μm. The mixture was mixed in a "Wig-L-Bug"intense vibrator, and subsequently dispersed in a three-roll mill. Thebulk thermal conductivity of the paste was measured at 45 W/MK, which ishigher than the bulk thermal conductivity of most organic basedcommercially available thermal paste by a factor of approximately 50times.

The thermal paste was applied to the same sample chip thermal testmodule as in Example I. The thermal paste was spread on the module, thensimultaneously squeezed and sheared to a thin, approximately 37 μmthick, layer. A chip dissipating a power density of 518 W/cm² exhibiteda temperature drop (T drop=T junction-T inlet=85°-23° C.) of 62° C.Interpolating the result to a temperature drop of 60° C. yields a powerdensity of 501 W/cm². Thus, the total thermal resistivity isThRy=12E-6Km² /W=12 K/mm² /W. The thermal resistivity is the temperaturedrop across the paste multiplied by the chip area and divided by thepower, expressed in units of ° C. per W/cm ². The total ThRy includesthe paste itself and all the other components of the thermal pathincluding the cooling hat. The cooling bath water flow was 8 cm³ /secover a cold plate having an area of 1.1 cm² .

Other examples exhibiting satisfactory cooling were 33 wt% Ga-In-Snternary eutectic and 67 wt% 10 μm particles coated with a gold layer andprepared according to the procedure described above.

While there have been described several liquid metal matrix thermalpastes, it will be apparent to those skilled in the art that variationsand modifications are possible without deviating from the broad scope ofthe invention which shall be limited solely by the scope of the claimsappended hereto.

What is claimed is:
 1. A thermal cooling system comprising:a componentto be cooled; a cooling system; and a liquid metal matrix thermal pastecomprising a dispersion of thermally conductive particles disposed in aliquid metal matrix selected from the group consisting of liquid galliumand its alloys, said particles being non-reactive with said liquid metalmatrix at a temperature below approximately 100° C., whereby thefraction and dispersion of the particles in the liquid metal matrix aresuch that the resulting paste has a predetermined thermal conductivityand permanently remains compliant, said paste being dispersed betweensaid component and said cooling system.
 2. A thermal cooling system asset forth in claim 1, wherein said cooling system comprises a piston andchip cooling system, and said paste is disposed between the piston andthe component.
 3. A thermal cooling system as set forth in claim 1,where said cooling system comprises a piston, cylinder and chip coolingsystem, and said paste is disposed between the piston and the cylinder.4. A method of preparing a liquid metal thermal paste comprising thesteps of:providing a liquid metal matrix selected from the groupconsisting of liquid gallium and its alloys; and dispersing thermallyconductive particles into said liquid metal, said particles beingnon-reactive with said liquid metal matrix at a temperature belowapproximately 100° C., whereby the fraction and dispersion of theparticles in the liquid metal matrix are such to form a mixture having apredetermined thermal conductivity and permanently remaining compliant.5. Method of preparing a liquid metal matrix thermal paste as set forthin claim 4, wherein said dispersing is by mixing and subsequent exposureto a roll mill.